Slag is the waste product which floats over molten steel within electric arc furnaces used in steel refining (e.g., smelting) operations. The main functions of the slag in the steel-making process include insulating the metallic bath (thereby helping to maintain the steel temperature), stabilizing the electric arc to protect the refractories from flare damage, improving the quality of the steel by absorbing sulfur and phosphorus, protecting the steel from oxidation, and other numerous benefits.
Conventional slag practice involved using high calcium lime, dolomitic lime, or a mixture of both to form the slag cover. However, due to the high temperatures generated by the electric arc within the furnace, there is a high loss of energy and notable chemical damage to the furnace refractory lining. To resolve this problem, the practice was to inject carbon over the bath to generate gases, such as carbon monoxide and carbon dioxide, that would be entrapped within the slag to form a foamy slag cover. This foamy slag cover improved heat transfer from the arc to the bath for faster temperature pickup of the metallic bath, resulting in improved heat transfer and reduction of the heat load on the furnace refractory walls and roof.
Canadian Patent Application No. 2,193,539 of Herrera-Gandara and Flores-Morales disclosed the use of a mixture of the mineral dolomite with mineral vermiculite, the latter of which is used for its ability to expand and foam the slag when exposed to the heat within the furnace. The dolomite and vermiculite crude ores are mined as raw minerals obtained by opencast working using explosives. The materials are then crushed using jaw crushers or roll crushers, and the crushed material is sized through a vibrating screen system to under 3/8 inches. The dolomite and vermiculite crude ores are mixed together in a 75:25 ratio and conveyed by hopper car to the electric arc furnace into which they are introduced. This mineral combination is said to improve physical and chemical properties as well as metallurgical functions of the slag by totally or partially replacing high calcium lime and/or dolomitic lime.
In contrast to the prior art processes, wherein crude (though size-screened) vermiculite particles are deployed in combination with other minerals, the present inventor believes that superior slag modification and steel refining performance can be achieved through the use of refined vermiculite flakes which can be used with other minerals but which are preferably deployed alone. A refined vermiculite product has been used for molten steel insulation (for example, it is commercially available under the MSI.RTM. brand name of W. R. Grace & Co.-Conn. through its Grace Construction Products unit), but this use of exfoliated vermiculite over molten steel is employed merely as thermal insulation in a downstream process and has not been used or suggested for use directly in the electric arc furnace smelting operation itself. The present inventor believes that the novel application of refined vermiculite, in the form of flakes, provides numerous advantages over the prior art.